1. FIELD OF THE INVENTION
The present invention relates generally to electronic measuring instruments, and in particular to the calibration of electronic measuring instruments having transducers that produce electrical signals in response to physical quantities and electronic circuitry that processes these signals.
2. THE PRIOR ART
Electronic instruments using transducers that produce electrical signals indicative of the magnitudes of physical quantities are known to the art. Examples of such instruments include electronic scales and gas flow controllers. Typically, a transducer employed by such an instrument is connected as an arm of a bridge circuit, and when a physical quantity to be measured (such as weight, gas flow, or the like) is sensed by the transducer, the bridge produces a signal indicative of the magnitude of the quantity being measured. This signal is amplified to yield an output signal that in turn can be used for such purposes as activation of a display or control of a process.
Calibration of an electronic measuring instrument is normally carried out by applying a plurality of physical quantities of known magnitudes to the transducer, one at a time, and adjusting the resulting output signal to desired levels corresponding with these known magnitudes. However, if the overall system response of the instrument is linear, then only two calibration points are needed to fix the origin and slope of a "calibration line" defining the relationship between the magnitude of the physical quantity being measured and the magnitude of the output signal. Zero is frequently chosen as a convenient reference point that can be used to determine the origin of the calibration line.
After initial calibration has been performed, the instrument can be expected to drift slowly out of calibration as time passes, due to aging of the components and other causes. Accordingly, periodic recalibration is necessary in order to preserve the initial accuracy of the instrument, and such recalibration must be done manually from time to time.
Although both the origin and the slope of the calibration line are subject to drift, drift of the origin away from zero, a drift manifested by the introduction of an error of constant magnitude into all the measurements, is the more common kind of drift and generally has the more serious impact on instrument accuracy. If such drift could be automatically compensated for, the accuracy of electronic measuring instruments would be significantly improved and the permissible time between successive recalibrations could be correspondingly increased without sacrificing instrument accuracy.
It will be apparent from the foregoing that there is a need for an electronic instrument that automatically compensates for drift in the origin of its calibration line. The present invention satisfies this need.